In the last twenty years, the rated power of wind turbines has been gradually increasing thanks to the increase in their rotor diameter, which in turn makes it indispensable to use taller towers. The increase in height can make it essential for the tower to comprise different sections throughout its height, which are placed stacked on top of each other to form the tower and which at the same time are transportable by road or rail. For example, to assemble a tower 100 m tall 5 sections of 20 m in height could be used being stackable on top of each other, while at the same time being transportable with said dimensions by road and rail.
On the other hand, one of the options for achieving taller towers being similarly stable and rigid is to increase the transverse dimensions of the tower gradually from the top part to the base of the tower. Said increase can imply again problems with transporting the sections and a common solution involves dividing them into longitudinal modules. The dimensions of the longitudinal modules allow for their transport by road or rail.
Logically, increasing the rated power of wind turbines entails increasing the weight and dimensions of all turbine components in general, with the following aspects being particularly relevant to the costs of assembly:                The height of the tower;        The diameter of the rotor and the weight of the blades-hub;        The weight of the nacelle and sub-components;        The weight and diameter of tower segments.        
Usually, the pre-assembly stage of sections is carried out in a zone near to the base of the tower, where given the weight of said sections and the irregularity of the ground, it is necessary to carry out civil works to set the ground up mostly by means of levelling and compacting operations, which entail an intensive use of materials and time.
The state of the art has a pre-assembly position for each section, side by side, either surrounding the base of the tower or forming a row to one side of it. These positions are sufficiently distant from each other so that the devices used for the pre-assembly can travel between the different sections during the stages of pre-assembly of each of them.
Also, it is common to use pre-assembly platforms disposed in said positions, on which the precast concrete segments that form a section are assembled, wherein generally one pre-assembly platform is used per section.
Once the precast concrete segments are placed on the pre-assembly platforms and have been adequately positioned to guarantee the section's dimensional tolerances, the vertical joints between the precast concrete segments are executed, normally in concrete.
Given the dimensions of the base of the sections and the space that must be disposed between them, the surface to be set up is large and therefore implies a high cost in human and material resources. This is not always possible as there are locations where the space available is limited due to the irregularity of the ground, such as in locations where the wind turbines are on a peak or close to a cliff.
The present invention solves all the above drawbacks by means of a method for assembling decreasing section concrete towers for wind turbines which comprises a phase of pre-assembly of concrete tower sections which allows to reduce the space required during the stages of pre-assembly of the different sections of the tower prior to lifting and consequently, the material used to carry out the civil works associated to setting said space up.